The present invention relates to a process for the recovery of uranium from wet-process phosphoric acid.
The wet-process phosphoric acid is the acid produced by contacting phosphate-containing rock with sulfuric acid and separating the calcium sulfate precipitated.
Natural phosphate rock generally contains from 0.01 to 0.02 weight percent of uranium. During the treatment with sulfuric acid, the major portion of the uranium is dissolved in the phosphoric acid. In spite of the concentrations of uranium contained in such phosphoric acid, it is feasible to recover this uranium owing to the large amounts of phosphoric acid manufactured in the world.
It is already known to recover the uranium contained in phosphoric acid by using a solvent. U.S. Pat. No. 2,859,094 whose disclosure is incorporated by reference herein, describes the use of a solvent mixture consisting of a dialkylphosphoric acid, in particular di(2-ethylhexyl) phosphoric acid, and of a trialkylphosphine oxide, in particular trioctylphosphine oxide. This mixture is dissolved in an inert organic solvent immiscible with water, such as benzene, n-heptane, n-octane, and preferably a mixture of hydrocarbons such as kerosene. In the process of recovering uranium values from phosphoric acid, the uranium is first oxidized to convert it to the +6 oxidation state. Then the phosphoric acid is contacted with the mixed solvent to obtain an aqueous phase consisting of essentially uranium-free phosphoric acid and an organic phase consisting of the solvent containing the major portion of uranium.
A process for the reextraction of the uranium from this solvent is described in ORNL-TM 2522 of April 1969 by F. J. Hurst, Oak Ridge National Laboratory, whose disclosure is incorporated by reference herein. According to this process, the uranium-containing organic solution is contacted with an aqueous ammonium hydroxide solution and with an ammonium carbonate solution. Most of the ammonium hydroxide solution is preferably fed to the first reextraction stage and most of the ammonium carbonate solution to the last stage of a countercurrent system. In other words, uranium is transferred to the aqueous phase where it forms a solution of ammonium uranyl tricarbonate. Simultaneously, iron extracted with the uranium precipitates in the aqueous phase as ferric hydroxide.
The process described in report ORNL-TM 2522 of F. J. Hurst has numerous drawbacks. The organic solvent, essentially freed of uranium by the reextraction process, contains, at equilibrium with the uranium-containing aqueous phase, an appreciable amount of ammonia and also an amount of water which is larger than the amount of water contained in the organic solvent used to extract the uranium from the phosphoric acid, which solvent also contains no ammonia. Thus, when this ammonia-containing organic solvent is recycled to the uranium extraction stage, the uranium-free phosphoric acid which is obtained, is contaminated by large amounts of ammonium ions which most often interfere in using phosphoric acid in further applications. Moreover, in the presence of the phosphoric acid in this step, the solvent yields its water and the obtained uranium-free acid is diluted. It is therefore necessary to reconcentrate it. Furthermore, by using this prior art process, it is not possible to recover the uranium in yields higher than 88 percent by weight.